1. Field of the Invention
The present invention relates to a printing blanket in a seamless cylindrical shape which is particularly suitable for use in high-speed web offset printing presses.
2. Description of the Prior Art
A printing blanket in a seamless cylindrical shape has been recently proposed as a printing blanket suitable for printing at all speeds from ordinary lithographic offset printing to high-speed printing done by a high-speed offset rotary printer or the like (e.g. Japanese Patent Laid-Open No. 301483/1993).
The above-mentioned printing blanket is formed by laminating a compressible layer comprising an elastomer such as rubber and formed in a porous and seamless cylindrical shape, a non-stretchable layer which is nonstretchable in the circumferential direction, and a surface printing layer comprising an elastomer formed in a seamless cylindrical shape in this order on the outer periphery of a cylindrical sleeve mounted on a blanket cylinder.
The non-stretchable layer is formed on the compressible layer by winding a thread (for example, a thread such as a cotton string) which is non-stretchable in a longitudinal direction and is easy to deform in a direction perpendicular to the longitudinal direction in helical fashion in the circumferential direction while applying a tensile force thereto.
Such a non-stretchable layer has the function of preventing the occurrence of a phenomenon whereby a compressible layer or the like released from a compressive force produced by a plate cylinder or the like after passing through a nip deformed portion caused when it is pressed by the plate cylinder or the like at the time of printing excessively expands in the radial direction by elastic rebound forming a so-called bulge. The non-stretchable layer prevents the occurrence of such a phenomenon whereby the surface printing layer forms waves by repeating the above-mentioned nip deformation and creates bulge at high speed, that is, a stationary wave.
Furthermore, the non-stretchable layer also has the function of applying a radial preload to the compressible layer in order to improve the reaction force produced in pressing and deforming the printing blanket in the radial direction when it is pressed by the plate cylinder or the like. The magnitude of the preload and the magnitude of the reaction force are greatly related to the printing properties of the printing blanket and particularly to dot or solid applicability (the properties of forming prints without white spots) or to the properties of preventing deformation of dots such as double or slurry.
Specifically, the dot or solid applicability depends on the magnitude of the reaction force of the printing blanket. The larger the preload applied by the non-stretchable layer and the larger the reaction force, the better the dot or solid applicability is made.
On the other hand, the properties of preventing the deformation of dots depend on the degree to which the bulge is absorbed ahead of and behind the nip deformed portion. The smaller the preload applied by the non-stretchable layer, and the smaller the reaction force of the printing blanket and the more easily the bulge is absorbed. Therefore, the properties of preventing the deformation of dots are improved. The reason for this is that the deformation of dots is mainly caused by the elongation in the circumferential direction of the surface printing layer by the occurrence of the bulge.
In the case where no preload is applied, or the case where the preload is too light, the properties of preventing the deformation of dots are improved, while the dot or solid applicability is degraded. On the contrary, in the where the preload is too heavy, the dot or solid applicability is improved, while the properties of preventing the deformation of dots such as double or slurry are degraded.
Therefore, the tensile force or the like in winding a thread composing the non-stretchable layer is established by considering the balance between both the properties, that is, by finding the range of the magnitude of the preload in which the balanced results of printing in which the dot or solid applicability is good and the deformation of dots is prevented are obtained, in such a manner that the preload in the above-mentioned range occurs when the printing blanket is fabricated.
"Double" means that dots slip, so that double printing is done, and easily occurs by the slip in the direction in which paper sheets are discharged. When such double occurs, dots become large, so that the color of a portion represented by the dots is made darker than that in the other portion.
"Slurry" is a phenomenon that dots are out of shape, so that dots warps having the shape of bristles or the tail of a comet.
In the printing blanket of the above-mentioned conventional construction, the preload always continues to be applied from the non-stretchable layer to the compressible layer even if it is not employed, whereby rubber composing the compressible layer easily causes stress relaxation due to compression set. Correspondingly, the preload is decreased by the non-stretchable layer and therefore, the dot or solid applicability is easily degraded, for example, due to the reduction in the reaction force of the printing blanket with time.
Therefore, in employing the above-mentioned printing blanket, printing conditions such as a pressing force (the amount of depression) of the plate cylinder or the like must be frequently adjusted in conformity to the degradation of the dot or solid applicability due to the reduction in the reaction force, that is, the amount of depression must be frequently increased in order to maintain the dot or solid applicability in a suitable state, for example.
Finally, at the time where the rubber composing the compressible layer causes compression set to such a degree that the degradation of the dot or solid applicability cannot be sufficiently covered only by adjusting the amount of depression or the like, or the degree of the deformation of dots such as double or slurry exceeds the allowable range because the amount of depression is too large to sufficiently absorb the bulge ahead of and behind the nip deformed portion, the printing blanket cannot be employed.